The invention concerns a method of hydroisomerizing paraffins emanating from the Fischer-Tropsch process. It particularly uses bifunctional zeolitic catalysts for hydroisomerizing paraffins emanating from the Fischer-Tropsch process, enabling highly upgradable products to be obtained, such as kerosene, gas oil and especially basic oils.
More particularly, the invention concerns a method of converting paraffins emanating from the Fischer-Tropsch process using a bifunctional catalyst containing a faujasite-type zeolite which may be specially modified, dispersed in a matrix generally based on alumina, silica, silica-alumina, alumina-boron oxide, magnesia, silica-magnesia, zirconia, titanium oxide or based on a combination of at least two of the preceding oxides, or based on a clay or a combination of the preceding oxides with clay. The special function of the matrix is to help to shape the zeolite, in other words, to produce it in the form of agglomerates, spheres, extrusions, pellets, etc. which can be put in an industrial reactor. The proportion of matrix in the catalyst is from 20 to 97% by weight and preferably from 50 to 97% by weight.
In the Fischer-Tropsch process the synthesis gas (CO+H2) is converted catalytically to oxygenated products and essentially linear hydrocarbons in gas, liquid or solid form. These products are generally free from heteroatomic impurities such as sulphur, nitrogen or metals. The products cannot, however, be used as they are, chiefly because their cold-resistance properties are incompatible with the normal uses of petroleum cuts. For example, the pour point of a linear hydrocarbon containing 30 carbon atoms per molecule (boiling point equal to about 450.degree. C., i.e., included in the oil cut) is about +67.degree. C., whereas certain specifications require a pour point below -9.degree. C. for commercial oils. These hydrocarbons from the Fischer-Tropsch process then have to be converted to more upgradable products, such as basic oils, after undergoing catalytic hydroisomerization reactions.
Catalysts which are currently used in hydroisomerization are all of the bifunctional type, combining an acid and a hydrogenating function. The acid function is provided by carriers of large surface area (generally 150 to 800 m.sup.2.g.sup.-1) which have surface acidity, such as halogenated (especially chlorinated or fluorinated) aluminas, combinations of boron oxides with aluminum, amorphous silica-aluminas and zeolites. The hydrogenating function is provided either by one or more metals from Group VIII of the Periodic Table, such as iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, or by a combination of a Group VI metal, such as chromium, molybdenum and tungsten, with at least one Group VIII metal.
Equilibrium between the acid and hydrogenating functions is the fundamental parameter governing the activity and selectivity of the catalyst. A weak acid function and a strong hydrogenating function give catalysts which are inactive and selective to isomerization, whereas a strong acid function and a weak hydrogenating function give catalysts which are very active and selective to cracking. A third possibility is to use a strong acid function and a strong hydrogenating function to obtain a catalyst which is very active but also very selective to isomerization. It is thus possible to adjust the dual activity/selectivity property of the catalyst by choosing each of the functions carefully.
Acid carriers - in increasing order of acidity - include aluminas, halogenated aluminas, amorphous silica-aluminas and zeolites.
Patent application EP 323 092 describes a catalyst comprising fluorine and platinum on an alumina carrier which is used in hydroisomerization.
Patent application EP 356 560 describes the preparation of a highly specific Y zeolite, which may be used in a catalyst for the Fischer-Tropsch Synthesizing reaction or in a hydrocracking catalyst.